Monocytes are phagocytes, which remove injured and dead cells, cell fragments, microorganisms, and insoluble particles from the blood and body tissues. Monocytes also secrete substances that affect the function of other cells, especially lymphocytes. They are produced in the bone marrow, and when mature are released into the peripheral blood. Although they do serve a phagocytic role in the blood, their main site of action is the body tissues.

The half-life for monocytes in the peripheral blood is approximately 8 hours. Monocytes migrate into the tissues, often to sites of inflammation, where they serve their primary purpose. Here they transform into fixed or free macrophages, and continue their function as avid phagocytes. When activated, macrophages may enlarge and have enhanced metabolism.

Monocytes provide defense against mycobacteria, fungi, bacteria, protozoa and viruses. They respond to chemotactic factors, phagocytize, and kill the microorganisms.